The present invention relates to a multiple torch type plasma generation device used for a plasma spray coating apparatus, artificial diamond manufacturing apparatus, cutting, jointing of metal and ceramics, reformation and surface treatment of substances, and the like, and a method of generating plasma in these apparatuses. More specifically, the present invention relates to an improved plasma generation technology used in a so-called plasma spray coating apparatus, and the like by which metal, ceramics and other substances are melted by, for example, a large current flowing in a gas, i.e., so-called arc, and plasma generated by the arc, and sprayed onto a target to be treated so that a rigid coating is formed thereon.
FIG. 9 shows main parts of a general purpose multiple torch type plasma spray coating device of prior art, wherein a main torch 1 is composed of an insulator 27 surrounding a main cathode 3, a main casing 4 having a discharge port, a second main casing 31, which surrounds the main casing 4, has a narrow port and is disposed coaxially with the main casing 4 through an insulator 29, and a main power supply 7 having a negative terminal connected to the main cathode 3 and a positive terminal connected to the main casing 4 and the second main casing 31 through switching means 8 and 34. A second main gas 33 to be used in the main torch 1 is supplied into the space defined between the main casing 4 and the second main casing 31 through a second gas supply port 32. Next, an auxiliary torch 2 is composed of an auxiliary torch start electrode 9, a first auxiliary casing 10, which surrounds the auxiliary torch start electrode 9, has a discharge port, and is mounted coaxially with the first torch start electrode 9 through an insulator 28, a second auxiliary casing 36 mounted coaxially with the auxiliary casing 10 through an insulator 30, and an auxiliary power supply 13 having a positive terminal connected to the positive terminal of the main power supply 7 and the auxiliary casing 10 of the auxiliary torch 2 and a negative terminal connected to the auxiliary torch start electrode through a switch means 14. An auxiliary gas 12 is supplied from an auxiliary gas supply port 11 and a second auxiliary gas 38 is supplied from a second auxiliary gas supply port 37.
The torches shown in FIG. 9 are started by the following sequence. First, the switch 8 is turned on so that the main power supply 7 causes main start arc 15 to be formed between the main cathode 3 and the discharge port of the main casing 4, and thus a main plasma gas 6 is heated to enable conducting plasma to be discharged from the extreme end of the main casing 4 of the main torch 1 through the narrow port of the second main casing 31. At the time, when the switch means 34 is turned on and then the switch means 8 is turned off, the main start arc 15 is extinguished by formed plasma, and at the same time the arc discharged from the extreme end of the main cathode 3 forms second main casing start arc 35, so that the main plasma gas 6 and the second main gas 33 are heated to enable a plasma flame 23 to be discharged to the outside of the main torch 1. Next, the switch means 14 is turned on so that the auxiliary power supply 13 enables auxiliary start arc 16 to be formed between the auxiliary casing 10 and the auxiliary torch start electrode 9, and thus the auxiliary gas 12 is heated by the arc to form conductive plasma to be discharged from the discharge port of the auxiliary casing 10. The conductive plasma is further discharged to the outside of the auxiliary torch 2 through the narrow port at the extreme end of the second auxiliary casing 36. Upon the completion of these processes, the conductive plasma discharged from the main torch 1 and the auxiliary torch 2 forms a conducting path, because these torches are disposed such that the axes thereof intersect each other. When the switches 34 and 14 are turned off at this stage, the main power supply 7 forms steady hair pin arc 17 from the extreme end of the main cathode 3 toward the outer surface of the discharge port of the auxiliary casing 10. When and amount of the gas to be supplied into the main torch and an amount of the gas to be supplied into the auxiliary torch 2 are adjusted, respectively at the time, the plasma flame 23 substantially aligned with the axis of the main torch 1 is formed, as shown in FIG. 9. At the time, the direction of the arc getting to the auxiliary casing 10 serving an anode of the steady hair pin arc 17 is substantially aligned with the axis of the auxiliary torch 2, but the arc is curved toward the direction in which the plasma flame 23 is discharge by being attracted thereby. As a result, the inner wall of the narrow portion of the second auxiliary casing 36 is partially damaged and a degree of the damage is increased as the plasma torch is operated for a longer time, and the plasma torch cannot be finally operated. Thus, a problem arises in that the plasma torch cannot be stably operated at a high output for a long time.
A thermal spray material 20, which is supplied toward the plasma flame 23 through a material supply tube 19, is quickly heated to a high temperature by high temperature laminar flow plasma 18 having a high enthalpy and melted, and goes to a substrate 25 without spreading to a wide area, by being accompanied with the plasma flame 23, as shown by fused spray particles 21. A plasma separation means 22 provided with a flame casing 26 and located just in front of the substrate 25 separates only the plasma 18 from the plasma flame 23 containing the fused spray particles 21, and the fused spray particles 21 comes into collision with the substrate just after the separation to thereby form a sprayed coating 24.
In the above description, the inner wall of each of the main casing 4, the second main casing 31, the auxiliary casing 10, and the second auxiliary casing 36 is arranged as a jacket, and thus the interior thereof is cooled by circulating water, and the like, but this arrangement is not shown in FIG. 9. Further, the cooling systems are omitted in the following description.